The present invention relates generally to launderable retroreflective products for use in personal articles such as articles of clothing. More particularly, the invention relates to retroreflective appliques that use a dielectric mirror to assist in retroreflection and to permit the applique to exhibit a variety of daytime colors.
The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein.
The use of retroreflective materials (sometimes referred to in the literature as reflective materials) such as retroreflective appliques to increase the visibility of pedestrians has long been known. Such materials have the property of reflecting incident light, such as light from a vehicle headlamp, back in the general direction from which the light originated, regardless of the angle at which the incident light impinges on the surface of the material. Thus, a person wearing such a material can be highly visible to drivers of such vehicles at night, depending on (i) the amount of retroreflective material used, and (ii) the reflectivity of the material.
The retroreflectivity is provided by a multitude of reflective facets arranged as cube corner elements, or, more commonly, by a layer of tiny glass beads or microspheres that cooperate with a specularly reflective material that is referred to herein as a mirror. In the latter case, the beads are partially embedded in a binder layer that holds the beads to a fabric or other substrate material, and partially exposed to the atmosphere. Incident light enters the exposed portion of a bead and is focused by the bead onto the mirror, which is disposed at the back of the bead embedded in the binder layer, whereupon the light is reflected back through the bead, exiting through the exposed portion in a direction opposite to the incident direction. This type of construction is referred to as xe2x80x9cexposed lensxe2x80x9d, because it uses microspheres with portions that are exposed to the atmosphere.
In exposed lens retroreflective appliques, a simple aluminum coating is the most commonly used mirror. Aluminum provides the article with good reflectivity for densely packed beads, and has good durability, but the aluminum is opaque to visible light and renders a xe2x80x9cgray-castxe2x80x9d daytime appearance. In applications where it is important that the applique exhibit other daytime colors, it is known to provide a dielectric layer or layers as the mirror. See, e.g., U.S. Pat. No. 3,700,305 (Bingham) or U.S. Pat. No. 4,763,985 (Bingham). The dielectric layer is partially reflective and partially transmissive to visible light. This characteristic permits the article to exhibit color by use of a colored binder layer or, if the binder layer is colorless, by use of a colored barrier coat, fabric, or other substrate. The color of the binder layer, fabric, or substrate is visible not only in the small spaces that may exist between neighboring beads, but also through the beads themselves.
Examples of known exposed lens dielectric mirror appliques will now be described with the aid of FIGS. 1a-d. In FIG. 1a, a monolayer of beads 10, also commonly referred to as microspheres, has been formed on a temporary carrier layer 12. The beads 10 are typically made of glass or ceramic, and have a refractive index of nominally about 1.9 but that can range from about 1.7 to 2.0. The beads typically have diameters of about 30 to 200 xcexcm, but the size of the beads is not considered critical. The beads 10 have been cascaded onto the carrier to form a monolayer of beads partially embedded in the carrier. In FIG. 1a, the carrier 12 is shown as a two-layer construction that consists of an upper layer 12a, in which the beads are partially embedded, and a lower layer 12b. The upper layer 12a is made of a heat-softenable composition to permit easy removal of the carrier 12 after application. In one known embodiment, layer 12a consists of polyethylene, and layer 12b consists polyethylene teraphthalate (PET). In another known embodiment, layer 12a consists of polyethylene and layer 12b consists of paper. A dielectric mirror is then formed on the exposed portions of the beads 10 by first depositing a layer 14 of sodium aluminum fluoride (Na3AlF6, also known as cryolite), followed by a layer 16 of zinc sulfide (ZnS). Cryolite has a typical refractive index of about 1.34 in the visible spectrum, which is low relative to the refractive index of ZnS, which is typically about 2.35. The optical thickness (i.e., the physical thickness multiplied by the refractive index) of each layer 14, 16 is approximately one-quarter wavelength of visible light for optimal reflective performance.
Next, as shown in FIG. 1b, a layer of bead bond material 18 is applied to the dielectric mirror formed by the layers 14,16. The bead bond layer maintains the integrity of the monolayer of beads 10 when the carrier 12 is later removed. An example of a known bead bond material 18 is a fluorescent colored polyester urethane.
To produce a finished transfer sheet, a polyurethane barrier coat layer 20 and a transfer adhesive layer 22 are applied in sequence as shown in FIG. 1c. The barrier coat layer 20 includes certain pigments to help provide the desired daytime color. The transfer adhesive layer 22 is a heat activated polyester adhesive so that the applique can easily be applied to a substrate of interest.
In FIG. 1d, the transfer sheet of FIG. 1c has been applied to a fabric 24 by application of pressure and heat, and the carrier 12 is shown being removed to expose the lower portions of the beads to air, thus permitting the applique to retroreflect light that is incident from below (from the perspective of FIG. 1d).
The applique just described can be sold in transfer sheet form (FIG. 1c) or instead in the more finished form of FIG. 1d. 
Another known exposed lens retroreflective applique is similar to that just described, except the bead bond layer 18 is a printable ink such as plastisol ink that functions not only as bead bond but also as a transfer adhesive, so that layers 20 and 22 are omitted. Further, such plastisol ink bead bond layer 18 can be screen printed in an image-wise pattern on the exposed portions of some beads 10 but not of others, after the dielectric mirror has been formed on the beads. FIG. 2 shows an example of a graphic 26 useable as the image-wise pattern for the applique. This type of applique is sometimes referred to in the art as a xe2x80x9ctransfer graphicxe2x80x9d. Plastisol inks that are used for this purpose are available from Plast-O-Meric Inc. (a subsidiary of The Geon Company, Avon Lake, Ohio) under item series SX864.
In known transfer graphic appliques, the bead bond 18 can be printed in an organic solvent-free form and then gelled by applying heat for a short period of time. This gelled state results from partial dissolution of the particles in the plasticizer and partial coalescence of the particles to form a weak image-wise film that is generally dry to the touch and can withstand mild rubbing without smearing. Once in the gelled state, the further application of heat during lamination causes the gelled plastisol to temporarily soften and flow and/or penetrate a desired substrate, such as fabric 24. During this process the particles are further dissolved by the plasticizer, and upon cooling, they are fused into a rugged image-wise film.
Transfer graphic appliques can be sold in transfer sheet form without bead bond (FIG. 1a), in a kit comprising such a transfer sheet and a separate supply of printable ink bead bond material, or instead in the more finished form of FIG. 1d. 
If the image-wise bead bond layer 18 is applied in the form of graphic 26 (FIG. 2), then once the carrier 12 is removed, the beads 10, dielectric mirror 14,16, and bead bond material 18 remain adhered to the substrate in the shape of the image-wise pattern or graphic 26. In those areas, the applique provides retroreflection of incident light under suitable nighttime lighting conditions, and provides daytime color by virtue of the dielectric mirror and colored bead bond material 18 under diffuse lighting conditions.
Often, exposed lens retroreflective appliques are applied to substrates that in use are subjected to repeated laundering. This category of substrates generally includes most articles of clothing. An extreme example is a fireman""s jacket that tends to get badly soiled on a frequent basis. Unfortunately, exposed lens dielectric mirror appliques have long beep known to exhibit a reflectivity that degrades from an initial value much more rapidly under repetitive laundering conditions than aluminum mirror appliques of like construction. For example, U.S. Pat. No. 5,837,347 (Marecki) describes an exposed lens dielectric mirror construction in Example 42 thereof that retains barely more than one-fourth of its initial reflectivity after 50 washings, and yet is characterized as a dramatic improvement over prior constructions. It is known that the amount of degradation can depend upon factors such as the composition and thickness of bead bond material used, the manner in which the bead bond is cured, and even the type of temporary carrier used (since it can impact the bead bond curing process). Nevertheless, testing has shown that typical exposed lens dielectric mirror appliques retain from about 5% to a maximum of only about 50% of their initial reflectivity after about 50 home laundering cycles.
Furthermore, currently available exposed lens dielectric mirror appliques, which use cryolite as a low index material, have a repetitive home laundering performance that varies considerably from lot to lot even for samples processed under seemingly identical conditions.
Therefore, an exposed lens dielectric mirror applique that could consistently exhibit excellent reflectivity retention even under repetitive home laundering conditions would be highly desirable.
The present inventors have discovered that using certain materials in the construction of the dielectric mirror in an exposed lens retroreflective applique can have a drastic and unexpected effect on the applique""s retained reflectivity under repetitive laundering conditions. For example, reflectivities of at least about 75% of an initial reflectivity, and even about 90% or greater, are achievable after as many as fifty home laundering cycles.
This application describes exposed lens retroreflective appliques that incorporate a dielectric mirror. The appliques exhibit an initial reflectivity. By appropriate materials selection, the appliques can be made to consistently retain at least about 75%, and in some instances greater than about 90%, of the initial reflectivity after fifty home laundering cycles. Further, the appliques usually retain at least about 90% of their initial reflectivity after the first twenty-five home laundering cycles. In one embodiment, the dielectric mirror of the applique includes zinc sulfide as a relatively high refractive index material, and calcium fluoride as a relatively low refractive index material. In another embodiment, the dielectric mirror includes zinc sulfide as the relatively high refractive index material and silicon dioxide as the relatively low refractive index material.